The present invention relates to dielectric filter devices for determining the frequency band pass characteristics to be used in mobile communications devices or the like.
It is generally known that monoblock-type dielectric filters are used in mobile communications devices for transmitting and receiving signals, for example, in the frequency band of hundreds of megahertz to several gigahertz, Mobile communications devices, such as portable telephones, in recent years are adapted to serve a multiplicity of functions and made more compact and lightweight, and many monoblock-type dielectric duplexers are singly adapted to process different transmission and receiving frequencies for use in such devices (for example, Japanese Patent No. 3205337).
An example of duplexer which is such a conventional dielectric filter of the monoblock type will be described with reference to FIG. 9. A dielectric block 10, for example, of a ceramic material or the like has hollow bores 40 to 44, 49, 50. Electrically conductive layers are formed on the outer peripheral side surface 20 of the block and the upper surface thereof around the openings of the bores as indicated at 33 to 39. Further provided on the upper surface are a first electrode 30, second electrode 31 and third electrode 32 which are separate from these conductive layers.
As shown in FIG. 9, the bores 40, 41 are arranged between the first electrode 30 and the second electrode 31. The bore 49 is positioned between the second electrode 31 and the peripheral side surface. The bores 42 to 44 are arranged between the first electrode 30 and the third electrode 32. The bore 50 is positioned between the third electrode 32 and the peripheral side surface 20. The first electrode 30 is connected to an antenna (not shown), the second electrode 31 to a transmitter, and the third electrode 32 to a receiver.
A band-pass filter for the desired transmission frequency band is provided by suitably adjusting the dimensions and shapes (e.g., the diameter and depth of bores, and distance between conductive layers) of the bores 40, 41 between the first and second electrodes 30, 31, the conductive layers 33, 34 in the vicinity of openings of these bores, and conductive layers 60, 62 joined to the outer peripheral side surface 20. A trap having an attenuation pole at the desired frequency in the vicinity of the above-mentioned transmission frequency band can be provided by suitably adjusting the dimensions and shapes of the bore 49 between the second electrode 31 and the conductive layer on the peripheral side surface 20, the conductive layer 38 in the vicinity of opening of the bore, and the conductive layer 62 joined to the peripheral side surface 20.
Similarly, a predetermined band-pass filter for the receiving frequency band is provided by suitably adjusting the dimensions and shapes of the bores 42 to 44 between the first and third electrodes 30, 32, the conductive layers 35 to 37 in the vicinity of openings of these bores and the conductive layers 60, 62 joined to the peripheral side surface 20. A trap having an attenuation pole in the vicinity of the receiving frequency band can be provided by suitably adjusting the shapes of the bore 50 between the third electrode 32 and the conductive layer on the peripheral side surface, the conductive layer 39 in the vicinity of opening of the bore and the conductive layer 62 joined to the peripheral side surface 20.
The dielectric duplexer described comprises two dielectric filters having two systems, i.e., transmitting system and receiving system, which use different frequencies. Since the filters are based on the same principles of the band-pass function and the trap function, these functions will be described in respect of the receiving system with reference to the filter characteristics diagrams of FIG. 8.
FIG. 8 shows the characteristics of the receiving band-pass filter which is capacitance-coupled by the bores provided between the first electrode and the third electrode and the conductive layers in the vicinity of these bores. The shapes of these bores, the conductive layers in the vicinity of the openings thereof and the conductive layer 61 joined to the peripheral side surface 20 are so adjusted as to provide the desired frequency pass band as illustrated. Although the function of trap afforded by the bore between the third electrode 32 and the peripheral conductor is so designed as to attenuate frequencies outside the pass band, it is extremely difficult to attenuate the entire higher frequency band outside the pass band. Thus, it is likely that a resonance mode such as a or b will project in the higher frequency attenuation band outside the pass band. This appears attributable to the presence of the second or third harmonic of the main resonance mode other than the main resonance mode which determines the pass band of the dielectric filter or dielectric duplexer, or to the presence of other resonance mode such as TM mode or TE mode. With communications devices, however, it is required that the attenuation characteristics of a higher band of the dielectric filter be reduced to the greatest possible extent.
In the field of mobile communications devices such as portable telephones, it is required in recent years that the parts be made ever smaller. Since mobile phones need to be highly portable as an important feature of the commercial product and must therefore be smaller in size, it is not desirable to provide improved filter characteristics by adding anew circuit components to the dielectric filter described.
To overcome the foregoing problems, the present invention provides a dielectric filter device comprising a dielectric block generally in the form of a rectangular parallelepiped and having a plurality of hollow bores formed therein and openings of the bores in an upper surface of the block, electrically conductive layers respectively covering a lower surface opposed to the upper surface, an outer peripheral side surface parallel to axes of the bores, inner peripheral surfaces defining the respective bores and the upper surface around the bore openings, and a plurality of electrodes separate from the conductive layers for connection to external devices, the dielectric filter device being characterized in that the conductive layer is removed in the form of a strip from at least one portion of the peripheral side surface.
In the dielectric filter device, the conductive layer removed portion of the peripheral side surface has one end continuous with the upper surface.
Further in the dielectric filter device, the conductive layer removed portion of the peripheral side surface has one end continuous with the upper surface and the other end continuous with the lower surface.
The dielectric filter can be given improved characteristics by the present invention without providing external parts on the filter.